Coleoptera

Beetles

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Introduction

The Coleoptera, or beetles, includes many commonly encountered insects such as ladybird beetles (family Coccinellidae), click beetles (Elateridae), scarabs (Scarabaeidae), and fireflies (Lampyridae). They live throughout the world (except Antarctica), but are most speciose in the tropics.

The oldest beetle fossils are from the Lower Permian (about 265 million years old; Ponomarenko, 1995); since then the group has diversified into many different forms. They range in size from minute featherwing beetles (Ptiliidae), adults of which are as small as 0.3 mm long, to the giant Goliath and Hercules beetles (Scarabaeidae), which can be well over 15 cm. While most species are phytophagous, many are predacious, or fungivores, or are parasitoids. They communicate to one another in many ways, either by use of chemicals (e.g. pheromones), sounds (e.g. stridulation), or by visual means (e.g. fireflies). They live in rainforest canopies, the driest deserts, in lakes, and above treeline on mountains.

In one sense the most unusual property of beetles is not some aspect of their structure or natural history, but their sheer number. There are more known species of Coleoptera than any other group of organisms, with over 350,000 described species. Perhaps the most famous quote about beetles comes from the great population geneticist J.B.S. Haldane, who was asked what might be learned about a Creator by examining the world. His response: "an inordinate fondness for beetles" (Fisher, 1988).

Characteristics

The most distinctive feature of beetles is the hardening of the forewings into elytra; it is from this that they get their formal name (koleos - sheath, pteron - wing). The elytra serve to protect the more delicate hind wings, as well as the dorsal surface of the abdomen, and may have been a key factor allowing them to exploit narrow passageways (for example, in leaf litter and under bark). During flight the forewings are opened enough to allow the hind wings to unfold and function:

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Other derived characteristics of beetles are:

hind wings folded under elytra, with reduced venation

hind two thoracic segments (mesothorax+metathorax=pterothorax) broadly connected with abdomen, so that the primary functional units of body are head / prothorax / pterothorax + abdomen, rather than the more typical head / thorax / abdomen of many other insects.

genitalia retracted into abdomen

adult antenna with 11 articles

Beetles are holometabolous insects, normally with adecticous, exarate pupae. Most species have chewing mouthparts. There is a gula present on the undersurface of the head.

The Suborders of Coleoptera

The four living suborders of beetles diverged from one another in the Permian and early Triassic, and are substantially different from one another. Adults differ in the structure of the prothorax, hind wing, abdomen, ovary, testes, and so on. The major differences are summarized in a table.

Polyphaga is by far the largest suborder, containing 85% of the known species, including rove beetles, scarabs, stag beetles, metallic wood-boring beetles, click beetles, fireflies, blister beetles, mealworms, ladybirds, leaf beetles, longhorn beetles, and weevils. Many are phytophagous. Adephaga includes ground beetles, tiger beetles, predacious diving beetles, and whirligig beetles; most adephagans are predacious. Myxophaga is a small suborder, containing less than 100 known species, whose members are small or minute, and associated with hygropetric habitats, drift material, or interstitial habitats among sand grains. Archostemata contains several families of beetles, most associated with wood; members of this family are somewhat similar to some of the earliest, Paleozoic beetle fossils.

Discussion of Phylogenetic Relationships

Compared to the other four large orders of insects (Hemiptera, Hymenoptera, Diptera, and Lepidoptera), the phylogenetic relationships of the major lineages of beetles are relatively poorly known. Only recently has some of the morphological data been examined phylogenetically (e.g., Beutel, 1997; Beutel and Haas, 2000), and molecular sequence information is only now being gathered.

There are several competing hypotheses regarding subordinal relationships. The two most widely discussed differ most strikingly in their placement of the suborder Polyphaga: this suborder is either the sister group of Myxophaga (Crowson, 1960, 1981; Machatschke, 1962; Klausnitzer, 1975; Beutel, 1997; Beutel and Haas, 2000), or the sister group of all remaining beetles (Lawrence and Newton, 1982; Kukalová-Peck and Lawrence, 1993), as shown in the following two figures:

Evidence for a close relationship of Polyphaga to Myxophaga includes the shared reduction in the number of larval leg articles (Crowson, 1960, 1981). Klausnitzer (1975) further considered the Adephaga as sister to Myxophaga + Polyphaga, based on completely sclerotized elytra, reduced number of crossveins in the hind wings, and folded (as opposed to rolled) hind wings of those three suborders.

Evidence for the alternative hypothesis, that Polyphaga is the sister group to remaining beetles, is based primarily on characters of wing structure, and on the loss of the cervical sclerites in the three suborders other than Polyphaga (Lawrence and Newton, 1982; Kukalová-Peck and Lawrence, 1993).

The composition of the clade Coleoptera is not in dispute, with the exception of the twisted-wing parasites, Strepsiptera. These odd insects have been regarded as related to the beetle families Rhipiphoridae and Meloidae, with which they share first instar larvae that are active, host-seeking triungulins and later instar larvae that are endoparasites of other insects (Crowson, 1981), or as the sister group of beetles (e.g. Kukulová-Peck and Lawrence, 1993), or more distantly related to insects (see further discussion in Strepsiptera).

Other Names for Coleoptera

Vernacular Names:
Beetles, Käfer, Escarabajo

References

Arnett, R.H. 1973. The Beetles of the United States (A manual for identification). The American Entomological Institute, Ann Arbor, Michigan.

Kirejtshuk, A.G. 1992. Evolution of mode of life as the basis for division of the beetles into groups of high taxonomic rank. Pp. 249-261 in M. Zunino, X. Bellés and M. Blas (eds.), Advances in Coleopterology. European Association of Coleopterology, Barcelona.

About This Page

Page: Tree of Life
Coleoptera. Beetles.
Authored by
David R. Maddison.
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First online 16 November 1994

Content changed 11 September 2000

Citing this page:

Maddison, David R.
2000. Coleoptera. Beetles.
Version 11 September 2000 (under construction). http://tolweb.org/Coleoptera/8221/2000.09.11in The Tree of Life Web Project, http://tolweb.org/

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